Unless you have some kind of custom gauge, your fuel gauge is just a ma meter. What that means is it senses current flow. The fuel sending unit is a variable resistance that is at it's smallest value when the tank is full, and has it's largest resistance when the tank is empty. Because your unit reads full all the time, either the inline dropping resistor is not correct or the tank sending unit or wiring is shorting to ground. This is not cause by a lack of a good ground connection.
